Image sensor and image reading apparatus

ABSTRACT

An object of this invention is to provide a structure which meets a high-quality reading requirement and realize high-speed color reading when the reading section of a color image forming apparatus adopts a color contact image sensor using CCDs as reading elements. In order to achieve this object, the image sensor of a color image reading section uses a color contact image sensor in which a plurality of CCDs are aligned as reading elements in the main scanning direction. In this case, each CCD has one analog shift register for RGB time-division reading, and three R, G, and B reading apertures arranged parallel to each other at a pitch corresponding to the reading resolution. The pixel pitch in the main scanning direction is constant.

This is a continuation of co-pending U.S. application Ser. No.10/192,126, filed Jul. 9, 2002, and entitled, “IMAGE SENSOR AND IMAGEREADING APPARATUS”. The disclosure of co-pending U.S. application Ser.No. 10/192,126 is hereby incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION Background of the Invention

Recent development of so-called digital color copying machines whichseparate a color document into colors, electrically read the document,print the obtained color image data on a sheet, and thus copy the colorimage is outstanding. With the popularization of digital color copyingsystems, demands have arisen for higher printing qualities of colorimages.

Particularly, a demand for printing black characters and thin blacklines more black and shaper is strong. When a black document isseparated into colors, yellow, magenta, cyan, and black color signalsare generated as signals which reproduce black, and data is printedbased on the obtained color signals. Black is reproduced by superposingthe four colors, and thus even slight misregistration between the colorssmears a thin black line. An originally black image cannot look black orblurs, greatly degrading the printing quality.

To increase the printing quality, there is proposed a method of, e.g.,extracting color information in an image signal or the feature of thespecial frequency such as a thin line or dot to detect the area of ablack character, color character, or the like. Further, the image isseparated into areas such as a halftone image area and dot image area,and the respective areas undergo corresponding processes. For example,if the area is a black character portion, the area is monochromated inblack.

To meet demands for cost reduction and reduction in power consumption ofthe light source, so-called contact image sensors (to be referred to asCISs hereinafter) are being utilized more and more. This CIS uses alight source such as an LED or xenon lamp, an equal-magnificationimaging optical system such as a SELFOC lens, and a CCD or CMOS linesensor, in addition to an image scanner using a reduction optical systemas a combination of CCDs and lenses.

For example, when a document is read by a color copying machine at areading rate of about 25 ipm (images/min), the reduction optical systemusing CCDs requires a document surface illuminance of about 30,000 lx.To the contrary, the examination results reveal that the use of thecontact image sensor needs only a document surface illuminance of about3,000 lx which is about 1/10 that of the reduction optical system, andcan offer the same S/N in reading at the same reading rate.

As a color CIS which reads 600 dpi at the equal magnification, CISs asshown in FIGS. 10 and 11 are proposed. FIG. 10 shows an example of thesensor pixel array of the color CIS. One 42-pm pixel corresponding to600 dpi is divided into three in the subscanning direction, and three R,G, and B reading lines are formed parallel to each other at a pixelpitch of ⅓.

There is also proposed an in-line sensor in which one 42-gm pixel isdivided into three in the main scanning direction and pixels are alignedin order of R, G, B, R, G, B, . . . , as shown in FIG. 11.

These two sensors, however, suffer the following problems.

The area of the reading aperture is ⅓ the pixel per color, and thesensitivity is low. As a result, the light quantity of a light sourcemust be directly increased, which poses a problem in increasing a speedof 30 sheets/min or more in a copying machine.

The reading position shifts every ⅓ pixel in the subscanning directionor main scanning direction in accordance with R, G, and B colorcomponents, and correction processing for alignment must be executed.Assuming that a G component exists at the center, R and B componentsundergo alignment processing using linear interpolation. Since linearinterpolation is smoothing processing, the MTFs of the R and Bcomponents become lower by about 30% than that of the G component.

This leads to a blurred image, and the black character determinationunit of the color copying machine tends to detect a thin black line asanother color owing to the MTF difference between the R, G, and Bcomponents of the image. The color copying machine having the blackcharacter determination function cannot determine a black character athigh precision, and the qualities of black characters and thin lines ona printed image are low.

Especially in the copying machine, the image quality is most importantin reading at the equal magnification of 100%, and then important inreading within the range of 70% to 140%. For this reason, the MTFdifference between R, G, and B components is undesirable in reading atthe equal magnification.

In an existing color 3-line CCD for a reduction optical system, theinterval between R, G, and B reading lines is an integer multiple of onepixel such as a pitch of four or two pixels. In reading at the equalmagnification, read data of respective colors are stored in a linememory and read out with a shift corresponding to the line interval.Only this processing can realize reading position correction processing.

A larger line interval, however, causes large color misregistration dueto mechanical vibrations. Similar to the MTF difference, a thin blackline is colored and leads to erroneous black color determination. Hence,the line interval is ideally one line. This also applies to a color CIS.

A general color CCD for a reduction optical system has R, G, and Breading aperture lines formed parallel to the main scanning direction.The color CCD, comprises two analog shift registers for chargetransfer/reading in correspondence with each aperture line, i.e., atotal of six analog shift registers. These analog shift registers do notpose any problem in the reduction optical system which uses only onecolor CCD.

In the color CIS, however, 16 color CCD chips are aligned in the mainscanning direction. The use of similar color CCDs results in 6×16=96reading outputs. Wiring and an analog processor circuit for processingthe outputs become bulky.

In the pixel structure of the CIS, 468 pixels are arranged on one chipof a CCD reading element for each color. If 16 chips are aligned,468×16=7,488 pixels are arranged in the main scanning direction. Forexample, to read an A4-size document in the longitudinal direction at 60dpi, there is proposed a structure in which the pixel pitch betweenpixels on each chip in the main scanning direction is decreased at theend, instead of an equal pixel pitch.

A change in pixel pitch does not generate any problem when the CIShaving this structure reads a document such as a printing paper sheethaving a document image with no periodicity. When a document image suchas dotted printed matter with periodicity is read, a change in pitch atthe end of the CCD reading element with respect to the spatial frequencyof the document image generates moire, and the density changes dependingon the phase.

The black character determination unit of a color copying machine adoptsan image area separation technique to check whether an area in processis a dotted image or character area. Also in this case, the pixel pitchis desirably equal.

SUMMARY OF THE INVENTION

The present invention has been made to overcome the conventionaldrawbacks, and has as its object to provide an optimum structure of acolor CIS for a high-speed copying machine. More specifically, it is anobject of the present invention to simultaneously rationally meetconditions: to realize high-speed reading while keeping a high S/N athigh sensitivity, to enable reading without generating any MTFdifference between R, G, and B colors, to minimize color misregistrationcaused by mechanical vibrations, to implement a color CIS with anappropriate circuit scale, and to accurately read a document havingimage periodicity.

To achieve the above object, according to the present invention, animage sensor is characterized by comprising image pickup element arrayson a plurality of lines that output signals of different colors for therespective lines, and a charge transfer unit on one line forsequentially reading out the signals of different colors from said imagepickup element arrays on the plurality of lines by time division.

Other objects and advantages besides those discussed above shall beapparent to those skilled in the art from the description of a preferredembodiment of the invention which follows. In the description, referenceis made to accompanying drawings, which form a part hereof, and whichillustrate an example of the invention. Such example, however, is notexhaustive of the various embodiments of the invention, and thereforereference is made to the claims which follow the description fordetermining the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing an image forming apparatus accordingto an embodiment of the present invention;

FIG. 2 is a sectional view showing a CIS;

FIG. 3 is a perspective view showing the structure of the CIS;

FIG. 4 is an enlarged view showing the pixel structure of a color CISaccording to the embodiment of the present invention;

FIG. 5 is a timing chart for explaining an image signal reading timingfrom a CCD according to the embodiment of the present invention;

FIG. 6 is a view macroscopically showing the color CIS;

FIG. 7 is a view for explaining the joint between CCD chips according tothe embodiment of the present invention;

FIG. 8 is a view for explaining subscanning operation;

FIG. 9 is a block diagram showing image processing according to theembodiment of the present invention;

FIG. 10 is a view for explaining a color CIS in which one pixel isdivided into three, R, G, and B colors in the subscanning direction; and

FIG. 11 is a view for explaining another color CIS in which one pixel isdivided into three, R, G, and B colors in the main scanning direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment according to the present invention will bedescribed in detail below with reference to the accompanying drawings.

FIG. 1 is a sectional view showing the arrangement of an image formingapparatus according to the embodiment of the present invention. In FIG.1, reference numeral 201 denotes an image scanner section which reads adocument and performs digital signal processing; and 200, a printersection which prints out on a sheet in full color an image correspondingto a document image read by the image scanner section 201.

A CIS module 202 incorporated in the image scanner section 201 will beexplained. FIG. 2 is a sectional view of the CIS module 202, and FIG. 3is an exploded perspective view thereof.

As shown in FIGS. 2 and 3, the CIS module 202 is constituted byattaching, to a mold 2026, a cover glass 2021, an illumination lightsource 2022 made up of an LED and light guide, an equal-magnificationimaging lens 2023 formed from a SELFOC lens or the like, a color linesensor 2024, and a board 2025 which holds the color line sensor 2024.

FIG. 4 is an enlarged view showing the microscopic portion of the colorline sensor 2024 incorporated in the CIS module 202. Each rectanglerepresents a photodiode serving as a reading pixel. The color linesensor 2024 is used for equal-magnification reading at 600 dpi, and thesize of the aperture of one pixel is about 34×34 pm.

Color filters of three, R, G, and B primary colors are formed on thephotodiodes of the color line sensor 2024.

Reference numeral 2024-1 denotes a light-receiving element array(photosensor) prepared by aligning photodiodes which are covered with Rfilters and transmit a red wavelength component of visible light.

Reference numerals 2024-2 and 2024-3 denote light-receiving elementarrays prepared by aligning corresponding photodiodes which are coveredwith G and B filters and transmit green and blue wavelength componentsof visible light. The three R, G, and B line form reading lines, whichgenerate charges—corresponding to incident light quantities on thephotodiodes during an accumulation time.

Reference numeral 2024-4 denotes a CCD analog shift register serving asa charge transfer unit for transferring charges accumulated in thephotodiodes 2024-1, 2024-2, and 2024-3; and 2024-5, an output amplifierfor converting a charge signal into a voltage and outputting the voltageas a voltage output signal.

The light-receiving element arrays 2024-1, 2024-2, and 2024-3 havingdifferent optical characteristics on the three lines are arrangedparallel to each other so as to read the same line of a document by theR, G, and B sensors. The CCD analog shift register 2024-4 is arrangedadjacent and parallel to the B light-receiving element array 2024-3outside the three light-receiving element arrays. The light-receivingelement arrays 2024-1, 2024-2, and 2024-3 and the CCD shift register2024-4 have a monolithic structure on a single silicon chip.

The photodiodes are arranged such that the pitch between pixels(distance between the centers of photodiodes) on each line in the mainscanning direction is set to about 42/gym. Also, the photodiodes arearranged such that the interval between lines is set to about 42 gm.

FIG. 5 is a timing chart for explaining the readout timing of an imagesignal from the color CIS.

Charges of R, G, and B colors for one line that are accumulated in thephotodiodes 2024-1, 2024-2, and 2024-3 during one line period (e.g., 350μs) are transferred at once to the CCD analog shift register 2024-4serving as a charge transfer unit in response to a shift pulse ΦSH atthe timing of the start of the next line.

The charges which have been output to the CCD analog shift register2024-4 and correspond to the respective pixels are sequentiallytransferred every pixel to the output amplifier 2024-5 in response to acharge transfer clock ΦM. The charges are converted into a voltage bythe output amplifier 2024-5 and output as a voltage output signal. Afterthe charge transfer unit 2024-4 transfers charges to the outputamplifier 2024-5, the charge transfer unit 2024-4 is reset by a resetpulse ΦRS.

First, dummy signals d1, d2, . . . , d6 are read out. Then, effectivesignals are read out for 468 pixels of each color in repetitive order ofgreen, blue, and read: G1, B1, R1; G2, B2, R2, . . . , G468, B468, andR468.

Signals are read out at these timings because the CCD analog shiftregister on one line common to the three light-receiving element arraystransfers charges of three colors in this embodiment.

FIG. 6 is a view macroscopically showing the color line sensor 2024.

Sixty sensor chips are aligned on a board 2024-6. Signals are outputfrom the respective sensor chips, and 16ch signals are simultaneouslyread out in correspondence with the chips. The readout 16ch signals aresubjected to gain offset adjustment by an analog signal processing unit101 (to be described later), and converted into digital signals by aninternal A/D converter. Since each sensor chip outputs only 1ch, thenumber of reading output channels can be decreased even with a pluralityof sensor chips, compared to the prior art. Wiring and an analogprocessor circuit for processing output image signals can be simplified.

FIG. 7 is an enlarged view showing joints between the 16 sensor chips.In FIG. 7, the charge transfer unit 2024-4 is not illustrated. In thisembodiment, the pixel pitch (distance between the centers ofphotodiodes) between pixels at the ends of adjacent sensor chips isdesigned double a normal pitch (pixel pitch within a single sensorchip).

In this arrangement, one pixel is omitted at the joint, but can berationally interpolated by preceding and subsequent pixels. Even animage such as a dotted image with a periodic property can be accuratelyread without generating moire or a density change.

The operation of the overall apparatus will be explained. In the imagescanner section 201 of FIG. 1, a document 204 set on a document glass(platen) 205 and pressed by a document press plate 203 is irradiatedwith light from the illumination light source 2022 incorporated in theCIS module 202 shown in FIG. 2. The light reflected by the document 204forms an image on the color line sensor 2024 via the lens 2023 in FIG.2.

The color line sensor 2024 separates optical information of the documentinto red (R), green (G), and blue (B) color components to read thedocument in full color. The color line sensor 2024 outputs the R, G, andB color signals to a signal processor 207.

Each of the reading sensor lines of the respective color components inthe color line sensor 2024 is formed from 7,500 pixels. The color linesensor 2024 can therefore read at a resolution of 600 dpi an A3-sizedocument 297 mm in the widthwise direction, which is the maximum sizeamong documents set on the document glass 205.

The CIS module 202 mechanically moves at a velocity V in a direction(subscanning direction) perpendicular to the electrical scanningdirection (main scanning direction) of the reading sensor line, therebyscanning the entire surface of the document 204.

FIG. 8 is a top view showing the image reading apparatus of theembodiment for explaining an arrangement which performs subscanningoperation.

The color CIS 202 is stored in a resin carriage 410. Slide members 419and 420 are attached to the two ends of the color CIS 202, and pressedagainst the platen glass by spring members (not shown) incorporated inthe carriage 410.

The movement of the carriage 410 is restricted by a linear guide 418 soas to move in only the subscanning direction. A timing belt 411 isconnected below the carriage 410. As the timing belt 411 moves, thecarriage 410 moves interlockingly.

A stepping motor 413 transmits its power to the carriage 410 via thetiming belt 411, a timing belt 412, and pulleys 414, 415, 416, and 417under the control of a CPU 131 (to be described later) serving as acontrol means. Then, the carriage 410 moves in the subscanningdirection.

Referring back to FIG. 1, a standard white plate 206 is white with analmost uniform reflecting characteristic to visible light. The standardwhite plate 206 corrects shading of R, G, and B data read by the colorline sensor 2024. Shading correction is performed for document readingdata output from the R, G, and B sensors 2024-1, 2024-2, and 2024-3 onthe basis of data obtained by reading the standard white plate 206.

The image signal processor 207 electrically processes a read signal,separates it into magenta (M), cyan (C), yellow (Y), and black (Bk)components, and sends them to the printer section 200. One of M, C, Y,and Bk components is transferred to the printer section 200 everydocument scanning (scan) of the image scanner section 201. By a total offour document scanning operations, image data of one sheet is printedout.

In the printer section 200, M, C, Y, and Bk image signals from the imagescanner section 201 are sent to a laser driver 212. The laser driver 212modulates and drives a semiconductor laser 213 in accordance with theimage signals. A laser beam emitted by the semiconductor laser 213 scansa photosensitive drum 217 via a polygon mirror 214, f-θ lens 215, andmirror 216.

The developing device is made up of a magenta developing unit 219, cyandeveloping unit 220, yellow developing unit 221, and black developingunit 222. The four developing units alternately contact thephotosensitive drum 217 to develop M, C, Y, and Bk electrostatic latentimages formed on the photosensitive drum 217 with corresponding toners.

A transfer drum 223 holds a sheet fed from a sheet cassette 224 or 225around the transfer drum 223 itself, and a toner image developed on thephotosensitive drum 217 is transferred to the sheet.

After toner images of four, M, C, Y, and Bk colors are sequentiallytransferred, the sheet is discharged via a fixing unit 226.

The image signal processor 207 will be described.

FIG. 9 is a block diagram showing the flow of an image signal in theimage signal processor 207 of the image scanner section 201 according tothe embodiment.

The image signal processor 207 executes various processes using aregister and memory under the control of the control means 131implemented by a CPU.

A clock generator 121 generates clocks for each pixel, and a mainscanning address counter 122 counts clocks from the clock generator 121to generate a pixel address output of one line. A decoder 123 decodesthe main scanning address from the main scanning address counter 122,and generates a line unit sensor driving signal such as a shift pulse orreset pulse, a signal VE representing an effective area in a 1-linereading signal output from the color image sensor, and a line syncsignal HSYNC. The main scanning address counter 122 is cleared by asignal HSYNC, and starts counting the main scanning address of the nextline.

Image signals OS1 to OS16 output from the CIS module 202 are input tothe analog signal processing unit 101. The analog signal processing unit101 analog-multiplexes the signals so as to assign OS1 to OS6 to chi,OS7 to OS12 to ch2, and OS13 to OS16 to ch3. After gain adjustment andoffset adjustment, the analog signal processing unit 101 converts thesignals into an 8-bit digital image signal.

The 8-bit digital image signal output from the analog signal processingunit 101 is separated into signals of R, G, and B color components by arearrangement unit 102. A shading correction unit 103 performs knownshading correction using a reading signal of a standard white plate 211for the signals of the respective colors.

An inter-line correction unit 104 corrects spatial shifts between theshading-corrected color signals in the subscanning direction.

In the CIS module 202 used in this embodiment, as shown in FIG. 4, thethree light-receiving element arrays 2024-1, 2024-2, and 2024-3 of thecolor image sensor 2024 are arranged parallel to each other at apredetermined distance (42 μm equal to the size of the aperture of onepixel in the subscanning direction). Since the R, G, and B lines shiftfrom each other by one pixel in the subscanning direction, data atdifferent positions are simultaneously read in the subscanningdirection. The image data must be corrected to the same position, andthus a known so-called 3-line correction technique is adopted forcorrection.

The 3-line correction is employed as an essential technique in the useof an existing color 3-line CCD. In general, an image signal (in thiscase, a B signal) of a previously read line is accumulated in a memoryand combined with image signals (in this case, R and G signals) ofsubsequently read lines. The R and G color signals are delayed by linesin the subscanning direction with respect to the B signal so as toadjust the R and G color signals to the B signal, thereby correcting thespatial shift.

In the color CIS used in the embodiment, the interval between the three,R, G, and B reading lines is one pixel pitch which is an integermultiple of the size of one pixel in the subscanning direction. This cansimplify correction processing. As far as the interval is an integermultiple of the size of one pixel in the subscanning direction, thelines may be arranged at an interval of two or three times (two or threepixels).

An output from the inter-line correction unit 104 is input to an inputmasking unit 106. In order to transform the RGB signal reading colorspace read by the CIS module 202 into an NTSC standard color space, theinput masking unit 106 executes the following matrix calculation:$\begin{bmatrix}{R\quad 4} \\{G\quad 4} \\{B\quad 4}\end{bmatrix} = {\begin{bmatrix}{a\quad 11} & {a\quad 12} & {a\quad 13} \\{a\quad 21} & {a\quad 22} & {a\quad 23} \\{a\quad 31} & {a\quad 32} & {a\quad 33}\end{bmatrix}\begin{bmatrix}{R\quad 3} \\{G\quad 3} \\{B\quad 3}\end{bmatrix}}$

Luminance signals R4, G4, and B4 output from the input masking unit 106are converted into density signals CO, MO, and YO by a lightquantity/density converter (LOG transformation unit) 107 formed from alookup table ROM.

A line delay memory 108 delays the image signals CO, MO, and YO by linesuntil determination signals UCR, FILTER, SEN, and the like generated bya black character determination unit 113 (to be described later) fromthe signals R4, G4, and B4. As a result, the image signals C1, M1, andY1 and a black character determination signal for the same pixel aresimultaneously input to a masking UCR circuit 109.

The masking UCR circuit 109 extracts a black signal (Bk) from the inputthree primary color signals Y1, M1, and C1. The masking UCR circuit 109performs calculation of correcting color smear of a recording coloringmaterial in a printer 212. Then, the masking UCR circuit 109sequentially outputs signals Y2, M2, C2, and Bk2 with a predeterminedbit width (8 bits) every reading operation.

A main scanning scaling circuit 110 performs enlargement/reductionprocessing in the main scanning direction for an image signal and blackcharacter determination signal by known interpolation calculation.

A spatial filter processing unit (output filter) 111 switches edgeemphasis and smoothing processing on the basis of a 2-bit signal FILTERfrom a LUT 117, and outputs the resultant signal to the printer 212.

The black character determination unit 113 will be explained. The basicoperation is to determine whether an area of interest in a read image isa character/line image area or a dotted image area. If the area isdetermined to be a character/line image area, the UCR 109 increases theblack amount to make black clear. The output filter 111 emphasizes theedge, and the printer 212 switches the number of printing lines to beoutput to a fine-image setting and prints a sharp, clear character/lineimage. If the area is determined to be a dotted area, the filter 111executes filtering so as to blur dots, and the printer 212 switches thenumber of printing lines for higher tone level reproducibility.

The black character determination unit 113 is disclosed in detail inJapanese Patent Laid-Open No. 7-203198 filed by the present applicant. Acharacter thickness determination unit 114 determines the thickness of acharacter/line image portion in an image. An edge detection unit 115obtains edge information of the character/line image, and a saturationdetermination unit 116 attains saturation information.

Then, 4-bit determination signals ZONE and FCH from the characterthickness determination unit 114, a 3-bit determination signal EDGE fromthe edge detection unit 115, and a 2-bit determination signal COL fromthe saturation determination unit 116 are input to the LUT (lookuptable) 117. In accordance with these 9-bit input signals, the LUT 117outputs a 3-bit signal UCR for controlling the masking UCR unit 109, a2-bit signal FILTER for controlling the output filter 111, and a 1-bitsignal SEN for controlling the printer 212.

In this way, the masking UCR unit 109, output filter unit 111, andprinter 212 are appropriately controlled in accordance with thedetermination results of the character thickness determination unit 114,edge detection unit 115, and saturation determination unit 116. Properimage processing can be achieved in accordance with whether the currentarea is a black character, i.e., character/line image area or a dottedimage area, and a clear printing result can be obtained.

The above-described embodiment of the present invention has proposed anoptimum structure of a color CIS for a high-speed copying machine. Withthe use of the proposed color CIS, high-speed reading can be realizedwith high sensitivity and high S/N. Further, reading with highreproducibility of a thin black character line can be performed withoutgenerating any MTF between R, G, and B colors.

Since the pitch between lines is decreased, color misregistration bymechanical vibrations can be minimized. An image having periodicity canalso be accurately read, and these improvements can be implemented witha proper circuit scale.

The present invention can be applied to a system constituted by aplurality of devices or an apparatus formed from one device.

As has been described above, this embodiment can realize high-speedreading at high sensitivity and high S/N by using a color contact imagesensor.

The present invention is not limited to the above embodiments andvarious changes and modifications can be made within the spirit andscope of the present invention. Therefore, to apprise the public of thescope of the present invention the following claims are made.

1. An image sensor comprising image pickup element arrays on a pluralityof lines that output signals of different colors for the respectivelines, and a CCD shift register on one line for sequentially reading outthe signals of different colors from said image pickup element arrays onthe plurality of lines.
 2. The sensor according to claim 1, wherein thesignals of the different colors are output at once from said imagepickup element arrays on the plurality of lines to said charge transferunit.
 3. The sensor according to claim 1, wherein the signals ofdifferent colors include R (red), G (green), and B (blue) color signals.4. The sensor according to claim 1, wherein said image pickup elementarrays include three lines, said CCD shift register is arranged parallelto said image pickup element arrays and outside said image pickupelement arrays on the three lines, and when of said image pickup elementarrays on the three lines, image pickup element arrays on first, second,and third lines are arranged from a position farthest from said CCDshift register, said CCD shift register sequentially outputs a signal ofa second color generated by said image pickup element array on thesecond line, a signal of a third color generated by said image pickupelement array on the third line, and a signal of a first color generatedby said image pickup element array on the first line.
 5. The sensoraccording to claim 4, wherein the first color is R (red), the secondcolor is G (green), and the third color is B (blue).
 6. The sensoraccording to claim 1, wherein a pixel pitch between said image pickupelement arrays is equal in a main scanning longitudinal direction. 7.The sensor according to claim 6, wherein said image pickup element isrectangular, and the pixel pitch is substantially equal to a lineinterval between said image pickup element arrays.
 8. The sensoraccording to claim 6, wherein said image pickup element array on eachline is formed by aligning a plurality of sensor chips in the mainscanning direction, and at joints between the plurality of sensor chips,a pitch between pixels positioned at ends of adjacent sensor chips isset double a pixel pitch within the sensor chip.
 9. The sensor accordingto claim 1, further comprising a light source which illuminates adocument, and an imaging lens which forms an image of the document onsaid image pickup element at the equal magnification.
 10. An imagereading apparatus comprising an image sensor having image pickup elementarrays on a plurality of lines that outputs signals of different colorsfor the respective lines, a CCD shift register on one line forsequentially reading out the signals of different colors from said imagepickup element arrays on the plurality of lines, a light source whichilluminates a document, and an imaging lens which forms an image of thedocument on said image pickup element at the equal magnification, and amoving unit which relatively moves the document and said image sensor.11. The apparatus according to claim 10, further comprising a signalprocessing unit which performs predetermined signal processing for animage signal output from said image sensor.
 12. The apparatus accordingto claim 11, wherein said signal processing unit performs blackcharacter determination processing.
 13. The apparatus according to claim10, wherein the signals of the different colors are output at once fromsaid image pickup element arrays on the plurality of lines to said CCDshift register.
 14. The apparatus according to claim 10, wherein thesignals of different colors include R (red), G (green), and B (blue)color signals.
 15. The apparatus according to claim 10, wherein saidimage pickup element arrays include three lines, said CCD shift registeris arranged parallel to said image pickup element arrays and outsidesaid image pickup element arrays on the three lines, and when of saidimage pickup element arrays on the three lines, image pickup elementarrays on first, second, and third lines are arranged from a positionfarthest from said CCD shift register, said CCD shift registersequentially outputs a signal of a second color generated by said imagepickup element array on the second line, a signal of a third colorgenerated by said image pickup element array on the third line, and asignal of a first color generated by said image pickup element array onthe first line.
 16. The apparatus according to claim 15, characterizedin that the first color is R (red), the second color is G (green), andthe third color is B (blue).
 17. The apparatus according to claim 10,wherein a pixel pitch between said image pickup element arrays is equalin a main scanning longitudinal direction.
 18. The apparatus accordingto claim 10, wherein said image pickup element is rectangular, and thepixel pitch is substantially equal to a line interval between said imagepickup element arrays.
 19. The apparatus according to claim 17, whereinsaid image pickup element array on each line is formed by aligning aplurality of sensor chips in the main scanning direction, and at jointsbetween the plurality of sensor chips, a pitch between pixels positionedat ends of adjacent sensor chips is set double a pixel pitch within thesensor chip.